Means known in the prior art to attenuate or absorb waves include fixed type breakwaters and mounting types wave absorbing blocks. When the former fixed type breakwater is constructed, a large rubble-mound, etc. is installed at the bottom of the water, on which a concrete breakwater has to be constructed. Therefore, a tremendous amount of materials is needed while the progress of the construction work greatly depends on weather conditions. Hence, there is a problem that much cost and time are spent.
To the contrary, a mounting type wave absorber block is prefabricated somewhere else without being affected by weather, etc., and then mounted in the water. Therefore, said type is more advantageous than the fixed type breakwater in terms of cost and time.
However, even with said conventional mounting type wave absorber blocks, it has to be estimated according to predicted conditions of waves how many wave absorber blocks are to be installed in a predetermined region. Consequently, it often occurs that an excess number of wave absorber blocks is counted. Furthermore, if the blocks encounter abnormally high waves due to a typhoon in a repeated manner, the wave absorber blocks are sometimes lost because of washing-away demanding to supplement the blocks. Hence, also with the mounting type the wave absorber blocks, there is a problem that maintenance cost as well as mounting cost are high.